Co-pending U.S. patent application Ser. No. 021,169, assigned to the assignee of this application and filed on Feb. 22, 1993, the specification of which is incorporated herein by reference, describes a belt molding having a decorative plastic portion and unitarily formed end caps at the ends of the molding. The preferred process described for manufacturing the molding includes a step in which polyvinyl chloride (PVC) is injection-molded onto a substrate having an extruded PVC layer adhesively bonded thereto. As described, however, it is possible to injection-mold directly onto a metal core or substrate treated with a suitable adhesive, a plastic skin and end caps.
During an injection-molding process, as is known to those skilled in the art, a vacuum is applied to the mold cavity to ensure that the cavity becomes entirely filled with the molten material. Such low pressures also induce the flow of material into the cavity, such materials, generally molten polymeric materials being fairly viscous and somewhat resistant to flow. In situations where a high-gloss PVC product is being molded, in which appearance is a consideration, it is important that in-fill of the cavity be relatively complete, else any visible flaw can lead to a rejection of the product.
Either of the injection-molding processes described above in connection with the automotive trim piece, whether or not the substrate includes an extruded plastic layer, is known in the injection-molding arts as an insert molding process. This is because the substrate is inserted into the mold cavity of the molding apparatus to partly define the cavity. It will thus be appreciated, given the conditions in which an injection-molding process takes place, that there is a low tolerance for deviations in the size and shape of the insert part. Improper flow of molten material around the part can lead to a defective part which may or may not be salvageable by trimming. In any case, if a part is rejected both the substrate and injection-molded plastic portion are discarded. In the situation pertaining to the automotive trim piece described above, in which the substrate is already a highly processed part, even prior to the injection-molding step, such wastage is highly undesirable.
It is a practice in the art of injection-molding to use multi-cavity molds where possible. A single molding machine having two or more molding cavities can thus be used to manufacture two or more products in a single injection-molding step, and thereby be utilized more effeciently. As a general rule, there is an inverse relationship between how complicated the shape of part to be injection-molded can be and the number of cavities that can be incorporated into the same mold. The use of an insert in an injection-molding process further complicates the matter in this regard. In the case of the automotive trim piece described in the aforementioned co-pending application, the use of a multi-cavity mold for injection-molding a skin onto a belt molding, by contributing to an increased product rejection rate has lead to a lower than expected increase in the production capacity of a given injection-molding machine.